1. Technical Field
The present invention relates to fuel delivery systems for stationary and propulsion gas turbine engines, and in particular to fuel pumping and metering devices for turbine engines.
2. Description of the Related Art
The high burn rates of gas turbine engines requires the fuel delivery system to be capable of rapidly and precisely metering fuel. Traditionally, fuel delivery systems for turbine engines, particularly those used for jet propulsion, have included a fuel pump, a pressure accumulator and a fuel metering device, all of which are separate components mounted on or near the engine and coupled to the engine and fuel source by suitable fuel lines. The accumulator operates to dampen pulsation or ripple in the fuel caused by the pump so that the metering device can accurately dispense the appropriate amount of fuel to the engine fuel atomizer. The use of multiple components is expensive and occupies space, which is especially limited for propulsion systems.
It is desirable to reduce the number of components in the fuel delivery by combining the fuel pump and metering device into one unit. However, such combined device must meet both the extreme pump and the metering requirements for turbine engines. Some of the attributes of a turbine engine fuel pump include the ability to pump particle contaminated fuel for an extended time period. It must have good dry lift capability and be able to operate with high vapor-to-liquid ratios at the pump inlet. Moreover, if no accumulator or fluid muffler is to be used, the pump must also be able to provide generally non-pulsating fuel flow. The requirements of a turbine engine metering device, particularly those used for jet propulsion, include low power consumption and low hysteresis to operate with high efficiency and low friction. The device must also have a high turn-down ratio to accurately meter a wide range of flow rates. Additionally, the device must be compact and have minimal internal leakage.
In the turbine industry, the fuel delivery systems typically employ gear pumps which create a pressure differential by moving the fuel through a series of intermeshing teeth running at a high frequency. Gear pumps consume a lot of power and leak internally and are therefore less than ideal for jet engine use. Moreover, due to reliability concerns, gear pumps used for propulsion applications typically are powered by an engine driven gear box (rather than an electric motor) and therefore must be coupled to a separate metering valve via suitable fuel lines, which increases expense and occupies additional space.
U.S. Pat. No. 6,371,740, assigned to the assignee of the present invention and hereby incorporated as though fully set forth herein, discloses a fuel metering pump for turbine engines. The metering pump employs a rotating face cam to alternately reciprocate a pair of actuators that in turn drive a pair of rolling diaphragms to pump and meter the fuel. The metering pump is specially designed to drive the pumping members at a constant velocity to minimize pressure ripple and thus provide essentially non-pulsed metering of the fuel. The rolling diaphragm design assists in keeping contaminants commonly found in jet fuel from degrading the working components of the metering pump.
While the aforesaid metering pump provides a marked improvement in accurate fuel metering at the high flow rates required for jet engines, the diaphragms have pressure limitations that can make the metering pump unsuitable for high pressure applications like jet engines. In particular, it can be necessary in some jet engine applications to achieve a pressure rise of over 500 psi. This pressure must be generated and maintained will metering the high flow rates required for sustained combustion, which can be 700 pph or more.
Accordingly, a high pressure metering pump is needed that can efficiently and accurately meter fuel at the high flow rates required for jet engines